This disclosure is directed to systems and methods for maintaining constant or desired hue across a range of tint variation for images formed in an image forming device.
Color images are commonly represented in image forming systems as a series of discrete separations. Each separation may include a set of color density signals for one color. Each separation includes all those portions of the color image having a particular color. When all of the separations are combined together, they form the full color image. Color density signals define digital gray or contone pixels. Pixels are the individually-colored picture elements. Pixels are associated with color values that vary in magnitude over a range from a minimum value to a maximum value with a number of gradients between, corresponding to a given bit density of the color imaging system. Pixel values are commonly represented by eight bits in a data structure (e.g., frame buffer), providing 256 shades of each color. A color may therefore be considered the combination (or recipe) of multiple color values for each pixel in the area where the color appears in an image, which when viewed together, present the combination color. Usually, image forming signals include four primary color signals, i.e., cyan (C), magenta (M), and yellow (Y), and a black (K) signal. Together, these CMYK signals may be considered the colorant signals in a particular image forming device. Because the use of CMYK is well understood in the art, further explanation regarding its use and function in rendering images need not be articulated here.
Two of the most widely known conventional methods, or “paths,” for spot color emulation, and/or spot color rendering, in image forming devices are basically understood as follows. In one method, spot colors can be described in the applicable page description language (“PDL”) such as PostScript, or PDF. During raster image processing (“RIP”) in the digital front end (“DFE”), spot colors are detected, and a predetermined CMYK recipe for rendering the spot color is retrieved from generally a pre-stored table of values for each corresponding spot color. This approach is referred as the “CMYK Path.”
In another method, instead of retrieving predetermined CMYK values directly from a pre-stored table, a device independent target is retrieved in a color space representative of the spot color and converted to a CMYK value for rendering the spot color image.
U.S. Pat. No. 6,637,849 B2 to Maltz, which is commonly assigned, and incorporated by reference herein in its entirety, describes conventional technology for specifying a color in a coordinate system. Maltz teaches that color may be measured in terms of three components which are often referred to as L*, a* and b*, which define parameters by which any color may be specified in the coordinate system. In an L*a*b* system, L* generally corresponds to a lightness-darkness scale value, a* generally corresponds to a red-green scale value, and b* generally corresponds to a yellow-blue scale value. Appropriate L*a*b* values, for emulating spot color, are looked up from a table which differs from the CMYK table in that the L*a*b* values are considered to be reasonably device independent. The L*a*b* values are converted to CMYK values to achieve the final CMYK recipe. This conversion process utilizes a color transformation mechanism in the RIP. This second approach is referred to as the “LAB Path.”
The final CMYK recipes may be different based on being obtained from the CMYK Path and the LAB Path. For full tint renderings of a specific spot color, each recipe, regardless of the path by which it was obtained, should render a spot color image that is visually identical, provided other steps involved in obtaining and rendering are or the other of the CMYK recipes are consistent, such as, for example, the image color conversion profile is accurate.
Each of the paths has certain advantages and disadvantages depending on the desired spot color rendering in a given image forming device. For example, the CMYK Path provides accurate solid spot color results and is relatively simple to implement. The difficulty is that CMYK Path systems tend to be less accurate in preserving hues for spot color objects at less than full tint values, e.g., a tint value below 1. As a result, spot color results are difficult to color manage across a spectrum of tint values. Additionally, CMYK systems tend to introduce contours and discontinuities when printing sweeps and images. Conversely, LAB Path systems, principally based on the complex nature of the color space value retrieval and conversion process, are slower than CMYK Path systems, but the LAB Path systems provide better opportunity for precise color (hue) management across a range of tints less than 1.